This invention relates to switch assembly structure for electrical machinery and more particularly to an improved switch assembly which protects the contact points of the switch from external contaminants which can be utilized in a number of types of electrical machinery, for example, in a radial arm saw, in conjunction with conventional centrifugal actuators for dynamoelectric machines. While the invention is described in detail with respect to the saw application, those skilled in the art will recognize the wider applicability of this invention.
As is well known in the art, a number of dynamoelectric machines, for example, capacitor start and split phase induction motors, utilize a first winding combination for the "starting" condition of a motor operation, and a second winding combination for the "run" condition of a motor operation. These motors include a stator assembly and a rotor assembly, the rotor assembly including a shaft and the stator assembly including a plurality of windings. The selective energization of the windings is used to generate suitable forces for rotating the rotor assembly in both "start" and "run" conditions. As also known in the art, a centrifugal actuator mounted on the shaft is utilized to move a switch arm of a switch assembly mounted on a terminal board between a first position and a second position to cause selective electrical energization of the winding combinations.
These types of switch assemblies are known to the art. For example, U.S. Pat. No. 4,034,173 to Crow et al., discloses a starting switch assembly integrally formed with a terminal connection board which is adapted to be actuated by a conventional centrifugal actuator. U.S. Pat. No. 4,686,401 to Gehrt, et al., discloses a starting switch assembly integrally formed with a terminal connection board also designed to selectively engage and disengage contacts for electrical machinery such as the windings for dynamoelectric machines.
Generally, however, motor switches of the prior art have obvious drawbacks in some applications. First, there are certain applications for dynamoelectric machines that place these machines in environments having contaminants in the air to which the machine is exposed. For example, switches are utilized in motors which drive table saws or radial arm saws. When employed in a radial arm saw, the dynamoelectric machine is exposed to contaminants such as sawdust in the air. Often, these machines exposed to high concentrations of saw dust and other particles in the air are plagued by motor failure. Motor failure occurs because the electrical switch used in association with the open motor fails due to dust accumulation between the switch contact points.
As previously explained, the motors employed in the radial arm saws are either split-phase or capacitor start induction run motors that conventionally employ centrifugal actuators that control the operations of the starting circuit of the motor. When the motor is not running, the actuator closes the switch so that an auxiliary or start winding is connected to a source of electrical energy while the actuator disconnects the start winding as the motor reaches operating speed. Conventional switch structures of the prior art employed in this environment often malfunction because the contaminants that enter the switch and accumulate between the contact points do not allow the switch to close.
A number of attempts have been made in prior art to eliminate switch failure problems. In particular, attempts have been made to enclose the starting switch contacts in order to protect them from the environment. While this has reduced the failure rate of open motors used in the above applications, it has not completely solved the problem. For example, U.S. Pat. No. 4,414,443 to Gehrt, discloses an environmentally protected switch construction preventing foreign matter from affecting switch contact operation. However, the protective structure in this patent requires the use of a series of elements including a first washer, a soft, compressible washer inboard of the first washer, a third metallic washer positioned inboard of the pliable washer, and a spring positioned inboard of the third metallic washer, to create a suitable arrangement that protects the contacts from outside contaminates. This protective construction requires a plurality of parts, and is sensitive to alignment.
U.S. Pat. No. 4,922,066 to Crow et al., discloses an environmentally protected switch for dynamoelectric machines requiring the use of a metallic cap which surrounds the contact point affixed to the movable switch arm, a rubber-like boot affixed to the second contact post extending upward and interconnected with the metallic cap which surrounds the first contact point. As long as the boot adheres to the cap and the second contact post, the integrity of the enclosed environments for the switch contacts is maintained. However, since the integrity of the environment for the contact switch is dependant upon the alignment of the first metal cap and the rubber boot, as well as the alignment of the rubber boot on the second contact post, the metal cap and the rubber boot can become misaligned thereby destroying the integrity of the environmental seal. Even where misalignment is not a problem, the construction is relatively expensive to produce.